Small And Moderate Earthquakes Focal Mechanism Solutions Measurement And Its Application In Earthquake Tendency Analysis

The earthquake is a phenomenon of geological tectonic movement; it is that the earth interior stress state of some place surpasses the rock intensity to have the destruction result. It is the key problems to study the distribution and change of earth’s crust stress field for understanding and predicting the place of earthquakes, because of most earthquakes in China within the range of earth’s crust.The current research methods of tectonic stress field mainly include geological structure analysis, seismic analysis, in-situ stress measurement, numerical simulation and experimental simulation method. The most direct method reflecting the crust stress state is studying focal mechanism of earthquake in the deep crust. Compared with the strong earthquakes with less quantify and concentrated distribution, the small and moderate earthquakes are numerous and widespread, which provide the possibility of detecting the crustal stress state of the study area.Yunnan-Sichuan lies in middle-south of south-north seismic belt, it is one of the most intense seismic activities area in China where had taken place eleven large earthquakes in the past30years. Study the effect of small earthquake focal mechanism in earthquake tendency can take advantage from those seismic data.In this project, on the basis of regional seismograms of body and surface wave, we obtained a seriers of source paremeters of more than70M>3.8earthquakes from2008to2012, respectively with the Cut and Paste method. We have calculated focal mechanism of1928M>3.0small and moderate earthquakes from2003to2012by Hash method on basis of P first motion and S amplitude ratio. At the same time, moderate and strong shocks of focal mechanism have been collected in1976to2012.Our research shows the focal mechanism solutions of small and moderate M≥3.0earthquakes are dominated by strike slip from2003to2012in Sichuan and Yunnan region, with some reverse and normal fault. M≥4.0earthquakes are dominated by strike slip which consistent with the regional background. Fault type of3.0≤M≤3.9earthquakes is desperate randomly.The regional field stress is inverted by the method of damp regional stress (SATSI) with1651reliable focal mechanism solutions.The maximum horizontal principal stress orientation of the tow side of the Hong-he fault tail are the same: NW-SE; in Sichuan-Yunnan rhombic block stress is divided into two different state by Lijiang-Xiaojinhe fault:stress orientation of south region is NW-SE, that of north region is NS; stress orientation has a rotating clockwise tendency from inner Tibetan plateau to the east border of Sichuan-Yunnan rhombic block; in the eastern boundary area, there is a gradually transition from NEE-SWW in the north to NW-SE in south. The maximum horizontal principal stress orientation is parallel to fault orientation western Yunnan and southwest Yunnan. We compared our result with Harvard CMT result. The result shows that our result is comparatively trusty.There is a certain relationship between the occurrence of strong earthquake and the changes of P-axis azimuth CV value of small and moderate earthquakes. P-axis azimuth CV value of small and moderate earthquakes has a process of rise-decline before Wenchuan8.0earthquake and Lunshan7.0earthquake. Both P axis azimuth CV values and apparent stress has same tendency changes, it objectively reflects the changes of stress field in the process of earthquake preparation。...